1. Field of the Invention
The present invention relates to a laser oscillator device for producing a high-power laser beam for cutting workpieces of metal or the like, and more particularly to a laser oscillator device arranged to prevent contamination of optical components of an axial-flow CO.sub.2 laser.
2. Description of the Related Arts
Axial-flow CO.sub.2, lasers are widely used as small-size high-power lasers for cutting workpieces of meta or the like. The axial-flow CO.sub.2, laser employs a Roots blower for circulating a laser gas at a high speed of about 100 m/sec in a laser tube, so that the laser power will be increased to a range from 500 to 1000 W per meter of the laser tube.
FIG. 4 of the accompanying drawings illustrates a conventional axial-flow CO.sub.2 laser which has a discharge tube 1, a total reflection mirror 2, and an output coupling mirror 3. A laser gas is forced to flow through the discharge tube 1 by a Roots blower 6. The laser gas as it flows out of the discharge tube 1 is cooled by a cooling unit 7. Electrodes 5a, 5b are disposed in intimate contact with the outer surface of the discharge tube 1. A high-frequency voltage which is high enough to cause electron capture is applied to the electrodes 5a, 5b by a pumping power supply 4 to generate a stable discharge in the discharge tube 1 to emit a laser beam 8.
Major problems of the conventional axial-flow CO.sub.2 laser are contamination of the optical components (i.e., the total reflection mirror 2 and the output coupling mirror 3) and reductions in the laser oscillation characteristics caused by such contamination, i.e., a laser power reduction and a mode deterioration. When the contaminated optical components are removed and observed in detail, it can be seen that the surfaces of evaporated films on the optical components are deteriorated. The laser power reduction is mainly caused by a reduction in the reflectivity of the total reflection mirror 2. It has been confirmed through experimentation and various calculations that when the reflectivity of the total reflection mirror in a 1KW CO.sub.2 laser with two folded beam paths is reduced by 1%, the laser power is reduced by about 10%. The mode deterioration results from an optical distortion of the output coupling mirror 3, and is particularly large when the output coupling mirror 3 is made of ZnSe. The mode deterioration is caused by heat produced by absorption of the laser beam by impurities deposited on the surface of the output coupling mirror 3.
The impurities which contaminate the optical components (i.e., the total reflection mirror 2 and the output coupling mirror 3) have been analyzed. FIG. 5 of the accompanying drawings shows a spectrum of infrared radiation absorption by the contaminated total reflection mirror 2. The horizontal axis of the graph of FIG. 5 represents a wave number, i.e., the number of waves per cm. The wavelength becomes longer to the right along the horizontal axis. The vertical axis of the graph indicates the transmittance of infrared radiation, the transmittance being represented by %. The lower the transmittance, the greater the absorption. A study of FIG. 5 shows that the infrared radiation is absorbed by CH.sub.2, CH.sub.3 in a range from 2850 to 2950 cm.sup.-1, by CH.sub.2, CH near 1460 cm.sup.-1, and by CH.sub.3, near 1370 cm.sup.-1. These impurities come from the oil refined from petroleum and used as gear oil in the Roots blower 6. The infrared radiation is also absorbed by Si--O--Si in a range from 1000 to 1200 cm.sup.-1, Si--O--Si being originated from silicone grease used as bearing grease in the Roots blower 6. The infrared radiation is also absorbed by an OH radical near 3320 cm.sup.-1. Though the OH radical cannot be found in new oil, it can always be observed in the old laser gas. C.dbd.O which absorbs the infrared radiation in a range from 1700 to 1750 cm.sup.-1 results from an oil component which has been oxidized by exposure to the laser beam. These oil components are not uniformly deposited on the entire surfaces of the optical components, but are scattered in an island pattern according to a microscopic observation. The Roots blower 6 is of such a structure that when oil components enter the impeller housing of the Roots blower 6, they are introduced into the laser gas. These oil components go, together with the laser gas, into all parts of the laser oscillator device, and cause the above problems especially when deposited on the optical components.